Hydraulic clutches are subject to an oil pressure in a piston cavity due to centrifugal force on the oil or hydraulic fluid in the cavity when the cavity is rotating. In the case of an inching clutch with shiftable gear ratios between the clutch engine, the centrifugal pressure varies with both engine speed and gear selection if the piston cavity is on the input side of the clutch. If the piston cavity is on the output side of the clutch then the centrifugal pressure varies with the transmission output speed which is generally proportional to vehicle speed.
The centrifugal pressure variations cause variations in the vehicle response at different engine speeds, ground speeds and gear selection. For example, in a transmission as described in referenced Application A having 18 forward speeds and 9 reverse speeds the clutch pedal position at the inching point varies with changes in engine speed when 9th gear is selected, the inching point being higher with decreasing engine speed and lower with increasing engine speed. Also, the inching point is higher if the transmission is shifted to a gear lower than 9th gear, and when trying to obtain smooth shifts when rolling in gears above 9th gear, the inching point moves down as ground speed increases. These centrifugal pressure variations also result in a less than ideal response when attempting to limit inching capability, as described in referenced Applications B and C to improve clutch durability.